Drum type mixers for the production of asphaltic concrete or the recycling of used asphaltic pavement are afflicted with two ills, the production of blue smoke and the emission of particulates. The first arises because asphaltic drum mixers are typically of the "direct-fired" kind, meaning that there is direct contact of the hot burner gases with the asphalt. The second results because dust and fines from the aggregate or recycle material are entrained in the stream of hot air and gases through the drum. Current Federal standards limit smoke, referred to as the "opacity", to 20 percent vision impairment and the emission of particulates to no more than 0.04 grains per dry standard cubic foot ("GR/DSCF"). The latter standard is easily achieved with current bag houses but the former standard is another matter entirely.
Opacity arises, it has been found, not from the asphalt in old pavement being recycled--its asphalt is too old, too brittle, and too oxidized to contribute much to the smoke problem. Rather opacity stems chiefly from the lighter fresh asphalt, whether that added to all virgin aggregate or that added to recycled pavement. Mysteriously, asphalts of the same penetration and the same viscosity--the two indices by which all asphalts are graded--can have totally different smoke points. An asphalt obtained from one source may have a lower or higher smoke point than an asphalt of identical grade obtained from another source. One simply cannot tell whether an asphalt will or will not abide by the standard until it is actually injected into the heat of the drum. Consequently, to thwart the opacity problem the asphalt injection point has been moved further and further downstream in the drum. But that in turn often unduly shortens the span over which the fresh asphalt is mixed with the aggregate or recycled pavement - unless the drum is lengthened to compensate. Lengthening the drum, however, adds to cost and, more critically, encumbers the portability of the drum mixer. This is important because the majority of drum mixers are portable. Another tack, also encumbering and costly, has been to empty the drum mixer into, in effect, a wholly separate drum or a pugmill and add the fresh asphalt there, out of the burner's stream.
So the primary objects of the present invention are to accommodate liquid asphalts of low smoke points without lengthening the drum and without the need for an additional drum, pugmill or the like, all in order to preserve portability of the plant and to minimize cost. Another object of the invention is to do so with components which can be supplied either as an option to a drum mixer on order or as a "retro-fit" kit or attachment for one already in the field. A further object of the invention is also to do so in a manner which allows the drum mixer to be operated in normal fashion when the smoke point of the asphalt used is high enough to meet the opacity standard.